This invention relates to opto-electrical devices, for example devices for emitting or detecting light.
One specific class of opto-electrical devices is those that use an organic material for light emission or detection. Light-emissive organic materials are described in PCT/WO090/13148 and U.S. Pat No. 4,539,507, the contents of both of which are incorporated herein by reference. The basic structure of these devices is a light-emissive organic layer, for instance a film of a poly(p-phenylenevinylene (xe2x80x9cPPVxe2x80x9d), sandwiched between two electrodes. One of the electrodes (the cathode) injects negative charge carriers (electrons) and the other electrode (the anode) injects positive charge carriers (holes). The electrons and holes combine in the organic layer generating photons. In PCT/WO90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline)aluminium (xe2x80x9cAlq3xe2x80x9d). In a practical device one of the electrodes is typically transparent, to allow the photons to escape the device.
FIG. 1 shows a typical cross-sectional structure of such an organic light-emissive device (xe2x80x9cOLEDxe2x80x9d). The OLED is typically fabricated on a glass or plastic substrate 1 coated with a transparent material such as indium-tin-oxide (xe2x80x9cITOxe2x80x9d) to form an anode 2. Such coated substrates are commercially available. The ITO-coated substrate is covered with at least a thin film of an electroluminescent organic material 3 and a final cathode layer 4, which is typically a metal or alloy.
Some particularly attractive applications of such devices are as displays in battery-powered units such as portable computers and mobile phones. Therefore, to extend the battery life of such units, there is a particularly strong need to increase the efficiency of the light-emissive devices. One route to improving efficiency is by careful choice and design of the light-emissive material itself. Another is by optimising the physical layout of the display. A third is by improving the conditions for charge injection into and charge recombination in the emissive layer.
To improve the conditions for charge injection into and charge recombination in the emissive layer it is known to include a charge transport layer of an organic material such as polystyrene sulphonic acid doped polyethylene dioxythiophene (xe2x80x9cPEDOT-PSSxe2x80x9d) between one or both of the electrodes and the emissive layer. A suitably chosen charge transport layer can enhance charge injection into the emissive layer and resist reverse flow of charge carriers, which favors charge recombination. It is also known to form the electrodes from materials having work functions that aid the desired flow of charge carriers. For example, a low work function material such as calcium or lithium is preferred as the cathode. PCT/WO97/08919 discloses a cathode formed of a magnesium:lithium alloy.
According to one aspect of the present invention there is provided an opto-electrical device comprising: an anode electrode; a cathode electrode; and an opto-electrically active region located between the electrodes; the cathode electrode including a first layer comprising a compound of a group 1, group 2 or transition metal; a second layer comprising a material having a work function below 3.5 eV; and a third layer spaced from the opto-electrically active region by the first and second layers and having a work function above 3.5 eV.
According to a second aspect of the present invention there is provided a method for forming an opto-electrical device, the method comprising: depositing an anode electrode; depositing over the anode electrode a region of an opto-electrically active material; depositing over the region of opto-electrically active material a layer comprising a compound of a group 1, group 2 or transition metal and a layer comprising a material having a work function below 3.5 eV; and depositing over those layers a material having a work function above 3.5 eV to form a third cathode layer.
The compound is preferably a compound of a group 1 or group 2 metal, especially a compound of a group 1 metal, such as lithium. The compound may, for example, be any of a halide (e.g. a fluoride), an oxide, a carbide or a nitride. Some of these compounds may be electrically conductive, others may be electrically insulative. The compound may be a complex of a group 1, group 2 or transition metal, especially an organic complex.
The first layer may be spaced from the opto-electrically active region by the second layer. Alternatively the second layer may be spaced from the opto-electrically active region by the first layer. The closer of the first and second layers to the opto-electrically active region is preferably adjacent that region or there may be one or more other layers (preferably electrically conductive layers) between the first layer and the opto-electrically active region. The optoelectrically active region is suitably in the form of a layer, preferably a layer of an opto-electrically active material. The opto-electrically active region is suitably active to emit light or to generate an electrical field in response to incident light. The device is preferably an electroluminescent device.
The second layer suitably comprises a metal selected from the group comprising: Li, Ba, Mg, Ca, Ce, Cs, Eu, Rb, K, Y, Sm, Na, Sm, Sr, Tb or Yb, or an alloy of two or more of those metals; or an alloy or one or more of those metals with another metal such as Al, Zr, Si, Sb, Sn, Zn, Mn, Ti, Cu, Co, W, Pb, In or Ag.
The thickness of the first layer is suitably between 10 and 150 xc3x85. The thickness of the second layer is suitably less than 1000 xc3x85, and preferably less than 500 xc3x85. The thickness of the second layer is suitably more than 40 xc3x85 or 100 xc3x85, and optionally more than 150 xc3x85 or 200 xc3x85. The thickness of the second layer is preferably in the range from 40 xc3x85 to 500 xc3x85.
The first layer preferably comprises more than 80%, more then 90%, or than 95% or most preferably more than 99% of the said compound. The first layer preferably consists essentially of the said compound. The said compound may have an effective work function in the device of less than 3.5 eV
The said material of which the second layer is comprised preferably has a work function of less than 3.5 eV, less than 3.4 eV, or less than 3.3 eV or less than 3.2 eV, or less than 3.2 eV or less than 3.1 eV or less than 3.0 eV. The second first preferably comprises more than 80%, more then 90%, or than 95% or most preferably more than 99% of that material. The second layer preferably consists essentially of that material.
The material of any layer of the cathode that is in contact with the opto-electrically active region preferably does not cause significant degradation of the material of the active region when the two are in contact. The material of any layer of the cathode that is not contact with the opto-electrically active region may be a material that is capable of causing degradation of the material of the active region when the two are in contact The said compound of the first layer may, when in contact with the material of the active region, form an intermediate state between that of the material of the active region and that of the said material of the second layer.
The third layer suitably comprises a material (a xe2x80x9chigher work function materialxe2x80x9d) having a higher work function than those of the first and second cathode layers. The work function of the higher work function material is preferably greater than 3.5 eV or more preferably greater than 4.0 eV. The higher work function material is suitably a metal. The higher work function material and/or the third layer itself preferably has an electrical conductivity greater than 105 (xcexa9.cm)xe2x88x921. The higher work function material is preferably Al, Cu, Ag, Au or Pt; or an alloy of two or more of those metals; or an alloy of one or more of those metals together with another metal, or an oxide such as tin oxide or indium-tin oxide. The thickness of the third layer is preferably in the range from 1000 xc3x85 to 10000 xc3x85, preferably in the range from 2000 xc3x85 to 6000 xc3x85, and most preferably around 4000 xc3x85. (xe2x80x9cF8xe2x80x9d) or (2,7-(9,9-di-n-octylfluorene)-3,6-Benzothiadiazole) (xe2x80x9cF8BTxe2x80x9d). Alternative materials include small molecule materials such as Alq3.
There may be one or more other layers in the device. There may be one or more charge transport layers (preferably of more or more organic materials) between the active region and one or other of the electrodes. The or each charge transport layer may suitably comprise one or more polymers such as polystyrene sulphonic acid doped polyethylene dioxythiophene (xe2x80x9cPEDOT-PSSxe2x80x9d), poly(2,7-(9,9-di-n-octylfluorene)-(1,4-.phenylene-(4-imino(benzoic acid))-1,4-phenylene-(4-imino (benzoic acid))-1,4-phenylene)) (xe2x80x9cBFAxe2x80x9d), polyaniline and PPV.
According to a second aspect of the present invention there is provided a method for forming an opto-electrical device, the method comprising: depositing an anode electrode; depositing over the anode electrode a region of an opto-electrically active material;, depositing over the region of opto-electrically active material a material having a work function below 3.5 eV to form a first cathode layer; and depositing over the first cathode layer another material having a work function below 3.5 eV to form a second cathode layer of a different composition from the first cathode layer.